After John Isner won an epic three-day match against Nicolas Mahut at Wimbledon last Thursday only to lose in straight sets to Thiemo de Bakker the next day, the tennis star needed a bit of cheering up. So friends took Isner to Butter Monday night after he came to town to tape “Late Show With David Letterman” earlier that day. “He was with a bunch of guys, and they were picking up women,” said a source. Although he wasn’t “messed up,” a spy said he was “enjoying his booze and the scene. They were dancing and having the best time flirting.”